The simplest way I can do is use ping, I can send a broadcast packet to everyone in a subnet, so that they can response back. Let say I am in subnet of 192.168.0.x and the broadcast IP is 192.168.0.255, I can do this:

ping -b 192.168.0.255

How I know the broadcast IP is 192.168.0.255? I can check with ifconfig.

ifconfig eth0 | grep Bcast

Some routers are configured to filter broadcast and multicast packets to prevent broadcast storm, if so, broadcast is useless.